1. Field of the Invention
The present invention relates to vapor etching of samples, such as semiconductor materials and/or substrates, and, more particularly, to a system and method of vapor etching a sample.
2. Description of Related Art
Vapor etching of semiconductor materials and/or substrates is accomplished using gases, such as xenon difluoride. Specifically, in xenon difluoride etching, the xenon difluoride gas reacts with solid materials, such as silicon and molybdenum, such that the materials are converted to a gas phase and removed. This removal of these materials is known as etching.
Adding non-etching gases to the xenon difluoride can offer improvements to the etch process as described by Kirt Reed Williams,“Micromachined Hot-Filament Vacuum Devices,” Ph.D. Dissertation, UC Berkeley, May 1997, p. 396, U.S. Pat. Nos. 6,409,876, and 6,290,864. The advantages of adding non-etchant gases to xenon difluoride etching gas are noted in U.S. Pat. No. 6,290,864 and include improved selectivity, which is the ratio of etching of the material to be etched versus those materials that are intended to remain, and uniformity. Increases in both of these parameters ultimately lead to improved yield.
A common approach to xenon difluoride etching is through the pulsed method of etching. Information regarding pulsed mode etching can be found in Chu, P. B.; J. T. Chen; R. Yeh; G. Lin; J. C. P. Huang; B. A. Warneke; K. S. J. Pister “Controlled PulseEtching with Xenon Difluoride”; 1997 International Conference on Solid State Sensors and Actuators—TRANSDUCERS '97, Chicago, USA, June 16-19, pp. 665-668. In the pulsed mode of etching, xenon difluoride is sublimated from a solid to a gas in an intermediate chamber, referred to as an expansion chamber, which can then be mixed with one or more other gases. The gas(es) in the expansion chamber can then flow into an etching chamber to etch the sample, referred to as the etching step. The main chamber is then emptied through a vacuum pump and this cycle, including the etching step, is referred to as an etching cycle. These cycles are repeated as necessary to achieve the desired amount of etching.
Alternatively, xenon difluoride etching can be accomplished using a continuous method, such as that described in U.S. Pat. No. 6,409,876, where a single reservoir is connected to a flow controller to provide a constant flow of xenon difluoride gas to the sample to be etched. In addition, a means of mixing an additional, inert, gas to the etch gas between the outlet side of the flow controller and the inlet of the chamber is described.
Adding an additional gas, typically an inert or minimally reacting gas, such as nitrogen, to the etching process must be accomplished keeping in mind the sublimation pressure of xenon difluoride. Often, the partial pressure of the additional, non-etching gas is higher than the sublimation pressure, which is the pressure below which xenon difluoride is a gas and above which it is a solid. At 25° C., the sublimation pressure of xenon difluoride is approximately 4 torr. It is not uncommon during pulsed etching to mix in high pressures of other gases, such as nitrogen, into the expansion chamber after the expansion chamber has been filled with a few torr of xenon difluoride, to high pressures such as 30 torr, However, in a continuous process, such as that described in U.S. Pat. No. 6,409,876, the pressure of the additional gas mixed into the xenon difluoride would have to be less than the pressure of the supplied xenon difluoride gas. The reason for this limitation is that additional gas pressures higher than the xenon difluoride pressure between the outlet of the flow controller and the inlet to the etching chamber would cause the xenon difluoride to stop flowing through the controller.
It would, therefore, be desirable to provide a system and method of allowing the substantially continuous flowing of xenon difluoride gas with mixture of high pressures of additional gases.